Ghirma Moges

LC−Biosensor System for the Determination of the Neurotoxin β-N-Oxalyl-l-α,β-diaminopropionic Acid

An analysis system is described for the determination of the neurotoxin β-N-oxalyl-l-α,β-diaminopropionic acid (β-ODAP). The system is based on liquid chromatographic separation of β-ODAP from interfering amino acids on an anion exchange column coupled with an amperometric enzyme electrode for the registration of β-ODAP. The electrode is based on a graphite rod modified with an Os(2+/3+) redox polymer cross-linked with l-glutamate oxidase and horseradish peroxidase. This LC-bienzyme electrode analytical system enabled monitoring of as low as 4 μM β-ODAP (injection volume 100 μL). The β-ODAP content in real grass pea samples was measured to range between 3.3 and 5.2 g kg(-)(1) in dry grass pea.

Glutamate oxidase advances the selective bioanalytical detection of the neurotoxic amino acid beta-ODAP in grass pea: A decade of progress

The search for an enzyme as a reagent for selective bioanalytical detection of the neurotoxic amino acid, β-N-oxalyl L-alpha, β-diaminopropionic acid, β-ODAP (found in grass pea, Lathyrus sativus) led to its redox catalytic reaction by glutamate oxidase (GluOx). Homogeneous kinetic studies and an immobilized GluOx reactor-based flow-injection assay were initially made for beta-ODAP with small immobilized GluOx/catalase glutamate destroying prereactors. The method was applied to examine the toxin content in processed grass pea. The kinetics and the equilibrium of the thermal isomerization of β-ODAP to the nontoxic isomer α-ODAP established that GluOx is specific to the neurotoxin. The first ever GluOx-based amperometric biosensor for liquid chromatography (LC) detection was reported in 1997. This biosensor coupled with a refractive index detector improved LC performance. The most recent work with GluOx resulted in MnO2-based screen-printed amperometric biosensor, with offline elimination of glutamate interference by glutamate decarboxylase. A single-shot chemiluminescent sensor developed for hydrogen peroxide is also proposed for β-ODAP with GluOx application. This decade of progress resulted from studies that included four Ph.D. (Ethiopia, Sweden, Austria), four M.Sc. (Ethiopia, Sweden) and Licentiate (Sweden) theses projects, plus one collaborative project in Sweden. The advances in grass pea research may be regarded as a model north–south cooperation for research and education.

Thermal isomerization of the neurotoxin β-N-oxalyl-l-α,β-diaminopropionic acid

Abstract The rate constants and reaction order for the conversion of the neurotoxin β-ODAP to α-ODAP have been determined, after off-line thermal treatment, using the recently developed flow injection system for the neurotoxin based on a glutamate oxidase reactor. The effects of the initial concentration of the toxin, temperature and pH on the kinetics and the equilibrium of the endothermic process were examined. The conversion followed a zero order rate law. The reaction rate at pH 7 increased by about two-fold for every 10° temperature rise. The rate at pH 2 was about 60% higher than at pH 7.

Tetrachloroferrate(III)-selective liquid membrane electrode based on crystal violet

An extract of crystal violet-tetrachloroferrate(III) in nitrobenzene was used to prepare a tetrachloroferrate(III)-selective liquid membrane electrode with a poly(vinyl chloride) support. The optimal conditions to determine 2.5 × 10−5 − 5.0 × 10−2M iron(III) as tetrachloroferrate(III) (anionic slope 56 mV/decade, detection limit 7.9 × 10−6M) were found to be 4.0–5.5.M total chloride in 0.75–1.5M hydrochloric acid. The electrode was reliably applied to determine iron in human blood, haematite and mineralized vitamin syrup by direct potentiometry, standard and sample additions as well as standard subtraction techniques.

Hexafluorotantalate(V)-selective coated graphite electrode based on Malachite Green

ABSTRACT An attempt was made to develop hexafluorotantalate(V)-selective coated graphite electrodes based on Malachite Green, Crystal Violet, and Ethyl Violet using 1-chloronaphthalene, tri-n-butyl phosphate, and dimethyl phthalate as plasticizers. The electrode prepared by coating Malachite Green-hexafluorotantalate(V) extract in 1-chloronaphthalene with a PVC matrix on a graphite rod was found to give a Nernstian response to hexafluorotantalate(V) in a wide concentration range. Potential measurements with the coated graphite electrode were made against a plastic sleeve HF-resistant Ag/AgCl external reference electrode. Optimum response of the electrode was observed in test solutions made 1 mol/L in both sulphuric and hydrofluoric acids. The concentration range, slope, and detection limit from the plot of E vs log [Ta(V)] were found to be 1.0 × 10−5 – 1.0 × 10−2 mol/L tantalum(V), 58.0 ± 1.5 mV/decade, and 3.98 × 10−6 mol/L tantalum(V), respectively. The relative standard deviation for six determinations...

Potentiometric determination of tanatalum with a hexafluorotantalate(V) -selective liquid membrane electrode

Abstract Brilliant Green-hexafluorotantalate(V) extract in nitrobenzene was used as electroactive material to prepare a hexafluorotantalate(V) -selective liquid membrane electrode with a PVC support. The electrode body was made from concentric polypropylene tubes which were joined by a cork. The liquid membrane electrode was connected with fluoride-selective electrode as internal reference and ground glass diaphragm ( Ag AgCl ) electrode as external reference electrode to obtain a complete cell assembly for the potential measurements. The concentrations of sulphuric and hydrofluoric acid, for the optimum response of the electrode to hexafluorotantalate(V), were found to be 1 M each in the test solutions. The electrode was found to respond to hexafluorotantalate(V) in the concentration range of 2.0 × 10−6−1.0 × 10−2 M tantalum(V) with a slope of − 59 mV per decade and detection limit of 3.7 × 10−7 M tantalum(V) within 20–120 s. The effects of forty diverse ions on the electrode response to the hexafluorotantalate(V) have been studied and the electrode has been found to be highly selective to hexafluorotantalate(V). The newly developed liquid membrane electrode has been applied successfully to the determination of tantalum in tantalite-columbite ores by direct, sample addition, standard addition, and Grans plot potentiometric techniques.

Glutamate Oxidase-Catalyzed Oxidation of β-N-Oxalyl-L-α,β-N-Diaminopropionic Acid (ß-ODAP), a Neurotoxin in the Seeds of Lathyrus Sativus

Abstract Seven enzymes were tested in a screening of catalytic activity towards the neurotoxic L-amino acid, s-ODAP. L-glutamate oxidase, GIOD, was found to be the only one that showed activity and its kinetic properties were examined by measuring the rate of formation of hydrogen peroxide which was monitored spectrophoto-metrically at 512 nm, using Trinders chromogenic reagent. The Michaelis constant, KM, for the toxin is 0.24 mM at pH 7, comparable with the reported value for glutamate (0.21 mM). The activity of the enzyme was, however, considerably lower (0.78%) than that of the main substrate. Ammonia formation in the reaction was confirmed by the consumption of nicotinamide adenine dinucleotide (NADH) during reduction of α-kctoglutarate in presence of L-glutamate dehydrogenase (GIDH) and catalase.

Determination of Tantalum with Hexafluorotantalate(V)-selective Coated Graphite Electrode

A hexafluorotantalate(V)-selective coated-graphite electrode was prepared by coating a graphite rod with brilliant green-hexafluorotantalate(V) extract in 1-chloronaphthalene in a PVC matrix. Potential measurements were made against an HF-resistant plastic sleeve (Ag/AgCl) external reference electrode. The concentrations of sulfuric and hydrofluoric acids, for the optimum response of the electrode to hexafluorotantalate(V), were found to be 1M each in the test solutions. The electrode responded to hexafluorotantalate(V) in the linear range 5.0 × 10−6-5.0 × 10−3M, with a slope of -58 mV per decade and detection limit of 8.0 × 10−7M within 5–15s. The relative standard deviation for six determinations of 1.0 × 10−4M tantalum(V) was 2%. The life-time of the electrode was 60 days. The effects of forty diverse ions on the electrode response to the hexafluorotantalate(V) were studied and the electrode was found to be highly selective to hexafluorotantalate(V). Niobium, the element that commonly occurs with tantalum ores, showed a very low level of interference. The newly developed coated-graphite electrode has been applied to the determination of tantalum in tantalite-columbite ores and several synthetic matrices by direct, sample addition, standard addition, and Grans plot potentiometric techniques with reasonable precision (2–4%) and accuracy.